Expression if Output = value then do some calculation

Hi All

Below is my report

I need to calculate if Output = Starts on Scheme then 

subtract value Output = Starts on Scheme - Left Early

i used the below expression

=switch(Fields!Output.Value= "Starts On Scheme",(Count(Fields!Claim_Period.Value) - Sum(Fields!reason4leaving.Value)))

this does the subtraction but only for those where Output.Value= "Starts On Scheme and the remaining outputs.value =0

In the attached diagram  it calculation for  2nd column only (1- 0 ) =1

Any help much appreciated

Thanks

Pradnya07